1. Heat & measurement of Temperature
Physics Notes
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2. Effects of heat Heat is a form of energy. Its SI units is joule, J.
Expansion:
Occurs when things are heated.
Heat energy is gained by the body and results in an increase in size.
Contraction: Occurs when things are cooled. Heat energy is lost by the body and results in a decrease in size.

3. Expansion of Solids Solids expand when heated
Different solids expand at different rates

4. Expansion of liquids Liquids expand when heated.
When heating water in a beaker, you will initially notice a dip in the water level before the water level rises.
Different liquids expand at different rates.

5. Expansion of gases Gases expand when heated.
For a same rise in temperature, gas expands more than liquid, and liquid expands more than solid.

6. Expansion and contraction of water
As temperature increases from 4OC to 100 OC, water expands.
But from 0OC to 4OC, water contracts, that is, its volume decreases.
At 4OC, water has the lowest volume and therefore it has the highest density (since density = Mass/Volume and mass is constant).

7. Expansion problem in solids
Railway – gaps are left along the rails to allow for expansion.
Bridge – one end on the bridge is fixed while a gap is left at the other end supported by rollers.
Pavement – small gaps between cement sections are filled with soft black bitumen.

8. Expansion problem in liquids and gases
Bottles containing liquids are never completely filled. This allow space for expansion when the surrounding temperature is higher.
Aerosol cans carry a warning - to be stored in cool places.

9. Application of expansion and contraction
Riveting
Rivets are applied when heated. Once cooled, they contract and bind the metal sheets together tightly.
Bimetallic strip
Bimetallic strips works on the principle that different metal expands / contracts at different rate for the same increase / decrease in temperature.

10. Application of bimetallic strip
Heating element control
Fire alarm

11. Application of expansion and contraction
Hot air balloons
Hot air balloons depend on the low density of the hot air to float.
The air in the balloon is heated up which causes its volume to increase. Therefore the density of the hot air decreases. As the air in the balloon is less dense than the surrounding air, the balloon floats

12. Temperature, Melting and Boiling points
Melting point is the temperature where solid changes to liquid
Boiling point is the temperature where liquid changes to gas
Temperature
is a measure of how hot or cold a body is.
Temperature of a substance is commonly measured in degree Celsius (ºC).
SI unit : Kelvin (K)
The temperature 0K is called the absolute zero because there is no other temperature below it
At 0K, all types of motion of particles in matter will stop and the kinetic energy is zero

13. Thermometers
Thermometers make use of a measurable physical property that varies with temperature.
A Thermometer measures temperature in degree Celsius (oC).
Ice point is the temperature at which pure ice melts at 0OC
Steam point is the temperature at which pure water boils under normal conditions at 100OC.
Liquid-in-glass – clinical thermometer
Purpose – Measure body temp
Range : 35oC – 42oC
A constriction serves to prevent the mercury thread from going back into the bulb after the thermometer is removed from the mouth.
Liquid-in-glass – laboratory thermometer
Purpose – For Laboratory Use
Range : -10oC – 110oC

14. Other thermometers Resistance thermometer Thermocouple thermometer
Purpose: Lab use, resistance of wire changes with changes in temperature
Range: -200°C to 1200°C
Thermocouple thermometer
Purpose: Lab use, strength of current varies with temperature
Range: 100°C to 1100°C

15. Advantages of thermocouple thermometer
The temperature range of a thermocouple depends on the type of metallic wires used
Generally, thermometers have a very wide range, from -200°C to 1700°C
The junctions have a low heat capacity so that they can respond quickly to a temperature change

16. Other thermometers Constant volume gas thermometer
Pressure of the gas at constant volume varies with changes in temperature
Range: 20 K to 1300 K
Bimetallic thermometer
Purpose – Measure hot oven/ molten metal
Ranges: 0 to 400OC

17. Other thermometers Liquid crystal thermometer
Chemical changes which bring about colour changes

18. Calibrating a thermometer
Calibration means making a scale
To calibrate a liquid-in-glass thermometer, the following steps are taken:
Mark the lower fixed point, 0°C. The thermometer bulb is placed in pure melting ice. When the mercury meniscus is steady, its position is marked as 0°C
Mark the upper fixed point, 100°C. The thermometer bulb is placed in a hypsometer so that it is surrounded by steam from boiling water at ordinary atmospheric pressure. When the mercury meniscus is steady, its position is marked as 100°C
Divide the fundamental interval. The distance on the stem of the thermometer between 0°C and 100°C is measured and divided into 100 equal parts. Each part represents 1°C

19. Features of the laboratory thermometer
The round glass of the stem acts as a magnifying glass, enabling the temperature to be read easily
The sensitivity of the thermometer can be increased by
The glass of the bulb can be made thin so that the rate of heat exchange between the liquid and its surroundings can be increased
The narrow bore allows a big change in the length of the mercury thread when there is a slight change in volume of the liquid in the bulb
The bulb is made bigger to increase the volume of liquid stored in it. This increase in volume leads to greater thermal expansion or contraction
Liquids having a large thermal expansion like mercury and alcohol are used

20. Mercury vs Alcohol thermometers
Silver
Colourless, usually dyed red
High thermal conductivity – can respond very quickly to temperature changes
Low thermal conductivity – takes a longer time to reach its surrounding temperature
Uniform expansion as the temperature increases uniformly
Non-uniform expansion despite a uniform increase in temperature
Does not wet glass
Wets glass and gives inaccurate readings
Boiling point at 360°C – high upper limit, able to measure high temperatures
Boiling point at 78°C – low upper limit, unable to read high temperatures
Freezing point at -39°C – unable to measure low temperatures
Freezing point at -115°C – able to measure low temperatures
Poisonous liquid
Safe liquid
Expensive
Cheap
Very dense liquid
Light liquid
Small thermal expansion – scale markings on the stem are very close to one anther, making reading difficult
Large thermal expansion – scale markings on stem are far apart, making reading easy

21. Properties of clinical thermometer
A short temperature range, from 35°C to 42°C
A constriction in the capillary tube just above the bulb prevents the mercury in the thread from falling back into the bulb to allow reading

22. Question
The length of the mercury thread in a thermometer is 2cm when it is in pure melting ice and 22cm when it is in steam above boiling water at ordinary atmospheric pressure. When the thermometer is placed in liquid X, the length of the mercury thread is 16cm. What is the temperature of the liquid X? Solution If the temperature of the liquid X is m, m = (14 x 100°C) / 20 = 70°C

23. Questions Resistance thermometers are normally used to measure the
Boiling point of water
Freezing point of water
Melting point of metal
Boiling point of tap water
2. A liquid-in-glass thermometer is more sensitive if it has
shorter length
Smaller bulb
Thicker walled glass bulb
Thinner walled glass bulb